1. Field of the Invention
The present invention relates to a thermal line printer for performing thermal recording by a thermal line head and a method of driving the thermal line printer.
2. Description of the Related Art
A thermal line printer using a thermal line head where a plurality of heating resistors are arranged in a linear manner for thermally recording an image, a letter or the like onto thermal paper having a predetermined size is well known. In general, the thermal line head in such a type of a printer is divided into a plurality of blocks to be driven and controlled. The reason why the printer head is thus divided into a plurality of blocks and driven is because a very large amount of consumption power is needed when the electricity is applied to all the heating resistors to be simultaneously driven. This leads to an enlargement of a power source device, resulting in an increase in cost.
In such a printer, in the case where it is driven at a high printing rate (a rate of the heating resistors to be heated and driven and a rate of a so-called black printing), or in the case where it is driven under a low temperature circumstance, a so-called xe2x80x9cstickingxe2x80x9d phenomenon takes place.
The sticking phenomenon is a condition in which the thermal paper would stick to the thermal head due to a high printing rate, causing non-uniformity in feeding the thermal paper.
The cause for occurrence of the sticking phenomenon will now be described in brief with reference to FIGS. 8 to 10. FIG. 8 is a cross-sectional view showing a schematic structure of the thermal paper, FIG. 9 is an explanatory view schematically showing a condition where the sticking phenomenon takes place in the overcoat layer, and FIG. 10 is a time chart showing an example of a method of driving the conventional thermal line printer.
First of all, as shown in FIG. 8, it should be noted that the thermal paper K has a structure in which a base paper 101 is coated with a thermally sensitive layer 102 and an overcoat layer 103. Then, when a heating resistor 10 of a thermal line head H supplied with electricity in response to a print command, it is heated and the thermally sensitive layer 102 reacts to develop color. In this case, the overcoat layer 103 is molten and solidified after the elapse of a predetermined period of time (for example, one millisecond), so that the surface of the heating resistor 10 of the thermal line head H is stuck to the thermal paper K (see FIG. 9). For this reason, it is impossible to precisely feed the thermal paper K, leading to non-uniformity in pitch and resulting in defects such as degradation in printing quality. This is called sticking phenomenon.
A process of the occurrence of the sticking phenomenon in the conventional thermal line printer will now be briefly described with reference to the time chart shown in FIG. 10.
In this example, the thermal line head of the printer is divided into six blocks (Block 1 to Block 6). Then, printing is performed in order in a time division manner in all the blocks during a period Tc between one step drive and another step drive by a stepping motor. In this case, since it takes a relatively long period of time t1 from the completion of the printing by the head of Block 6 till the motor is step driven next, the overcoat layer 103 of the thermal printer K that has been once molten by heating of the head of Block 1 or the heads of Block 2 to Block 5 onward is cooled and solidified to cause the above-described sticking phenomenon, resulting in defects such as degradation in quality of printing.
Therefore, as shown in FIG. 11, there is an approach to apply a short pulse to every block again for the purpose of melting the overcoat layer after the completion of supply of electricity (Japanese Patent Application Laid-Open No. Hei 10-109435).
However, according to this method, since the pulse is applied in order to each block, a period of time t1xe2x80x2 from the application of the pulse to the final block until the paper feed exceeds the solidification necessary time of one millisecond in the first block so that the overcoat layer is solidified to cause the sticking phenomenon, disadvantageously.
The present invention has been made to solve the above problems, and an object of the present invention is therefore to provide a thermal line printer and a method of driving the thermal line printer, which may suppress the degradation in printing quality such as non-uniformity in density caused by the sticking phenomenon.
In order to attain this and other objects, according to the present invention, there is provided a method of driving a thermal line printer for performing thermal recording onto thermal paper to be fed by a drive of a stepping motor with a thermal line head where a plurality of heating resistors are arranged in a linear manner on a line perpendicular to a paper feeding direction, the method comprising the steps of: dividing the thermal resistors of the thermal line head into n blocks (where n is an integer not smaller than one), and further constituting m groups (where m is an integer not smaller than zero) from the n blocks; and driving each of the groups in order, and driving the stepping motor after each drive to thereby perform paper feed by less than one dot.
According to this method, the paper feed is performed immediately after the drive of each group of the thermal line head to thereby make it possible to prevent the sticking phenomenon.
It is preferable to drive in a time division manner the heating resistors to be driven in each block of the groups in order with a predetermined number of divisions in the case where the size of data to be printed in each group of the thermal line head is equal to or larger than a predetermined value.
According to this method, the power to be consumed in driving the thermal line head can be kept from exceeding a fixed level.
It is preferable that the number of divisions of the time division driving is determined in accordance with a temperature of the thermal line head. With this method, it is possible to generate a suitable amount of heat from the thermal line head.
Also, it is preferable to simultaneously drive the heating resistors to be driven in each block of the groups continuously for a predetermined period of time in the case where the size of data to be printed in each group of the thermal line head is equal to or less than the predetermined value.
With this method, the n blocks forming the groups are simultaneously driven to thereby make it possible to enhance the processing speed while suppressing the consumption power down to a fixed level.
Also, according to the present invention, there is provided a thermal line printer provided with a paper feed means for transferring thermal paper by a drive of a stepping motor and a thermal line head where a plurality of heating resistors are arranged in a linear manner on a line perpendicular to a paper feed direction, comprising: a thermal line head dividing means for dividing the thermal resistors of the thermal line head into n blocks (where n is an integer not smaller than one), and further constituting m groups (where m is an integer not smaller than zero) from the n blocks; and a control means for driving each of the groups and driving the stepping motor after each drive, to thereby perform paper feed by less than one dot.
Also, it is preferable that the control means comprises: a judging means for judging whether the size of data to be printed in each group of the thermal line head is equal to or larger than a predetermined value; and time division driving means for driving in a time division manner the heating resistors to be driven in each block of the groups in order with a predetermined number of divisions in the case where the size of data to be printed in each group of the thermal line head is equal to or larger than a predetermined value.
Furthermore, it is preferable that the control means comprises a division number determining means for determining the number of divisions of the time division driving on the basis of a temperature of the thermal line head.
Also, it is preferable that the control means comprises: a judging means for judging whether the size of data to be printed in each group of the thermal line head is equal to or less than a predetermined level; and a simultaneous drive means for simultaneously driving the heating resistors to be driven in each block of the groups continuously for a predetermined period of time in the case where the size of data to be printed in each group of the thermal line head is equal to or less than the predetermined value.